As of the 25th of september I landed bug 1263722 which adds a
PerformanceNavigationTiming entry to the existing performance API. The
implementation had a few issues which were fixed in bug 1403926, which also
puts this feature behind a pref: dom.enable_performance_navigation_timing
Right now this feature is on by default on all platforms. Other UAs
shipping this are Chrome and Edge.
Work on Navigation Timing 2 is tracked in bug 1043083. If you find any
issues, please file bugs blocking it.
Link to standard: https://www.w3.org/TR/navigation-timing-2/
We have web-platform tests covering this. I'm considering adding more, to
ensure proper order, and sane values for all fields - mostly issues fixed
in bug 1403926.